In a two-stage or single-stage power factor correction (PFC) circuit, generally, the product of a sampled output voltage of a rectifier bridge and a feedback signal of an output terminal of the PFC circuit is used as a reference signal of a current of a switch in the PFC circuit.
In this way, the input current in the PFC circuit changes with the input voltage, and for the entire circuit, and the phase of the input current Iin (at an alternating current input terminal) is the same as that of the input voltage Vin. FIG. 1 is a schematic diagram which shows an ideal case that the phase of the input current (at the alternating current input terminal) is the same as that of the output voltage.
To reduce high frequency pollution on an electric network, in conventional technology, generally a filter capacitor is connected to an input terminals or output terminals of the rectifier bridge in parallel, alternatively, filter multiple capacitors are connected to the input terminals and the output terminals of the rectifier respectively in parallel. However, the parallel connection of the filter capacitor may result in a phase difference between the input current Iin (at the alternating input terminal) and the input voltage Vin of the entire circuit, which is caused by the fact that the input current Iin (at the alternating input terminal) of the entire circuit is a sum of a current of the filter capacitor and the input current of the PFC circuit, and the phase of the current of the filter capacitor is heads of the phase of the input voltage by 90 degrees, i.e., due to the filter capacitor, the phase of the input current Iin leads the phase of the input voltage Vin.
Thus the parallel connection of the filter capacitor may result in different phases of the input current Iin and the input voltage Vin, thereby reducing power factor of the power source and increasing harmonic content.
In view of above, how to keep consistency between the phase of the input current Iin and that of the input voltage Vin is a technical problem to be solved by those of skills in the art.